[PATCH] aarch64: Make elf_machine_{load_address, dynamic} robust [BZ #28203]

[Fangrui Song]

On 2021-08-09, Szabolcs Nagy wrote:
>The 08/06/2021 11:44, Fangrui Song via Libc-alpha wrote:
>> The AArch64 ABI is largely platform agnostic and does not specify
>> _GLOBAL_OFFSET_TABLE_[0] ([1]). glibc ld.so turns out to be probably the
>> only user of _GLOBAL_OFFSET_TABLE_[0] and GNU ld defines the value
>> to the link-time address _DYNAMIC. [2]
>>
>> In 2012, __ehdr_start was implemented in GNU ld and gold.
>> Using adrp+addr to access __ehdr_start/_DYNAMIC gives us a robust way to
>> get the load address and the link-time address of _DYNAMIC.
>
>typo: adrp+add
>
>the binutils version is 2.23, i'd add that too to
>the commit message.

Done.

>this change only affects ld.so and static PIE and
>in both cases we can rely on newer binutils.
>(e.g. in libc_nonshared.a or crt1.o we can't)

Thanks for additional context.

>>
>> With https://sourceware.org/pipermail/libc-alpha/2021-August/129864.html,
>> this patch, and disabling traditional TLSGD tests (neither Clang nor
>> LLD's aarch64 port supports), LLD linked glibc has the same number of
>> `make check` failures.
>>
>> With this, I think
>> https://github.com/ARM-software/abi-aa/pull/57
>> doesn't need to specify _GLOBAL_OFFSET_TABLE_[0].
>> musl, FreeBSD, and NetBSD don't use _GLOBAL_OFFSET_TABLE_[0].
>
>i don't know who might be using GOT[0], but even if
>it's only glibc, we want future binutils to be able
>to build a working older glibc for a while so i don't
>think we can repurpose GOT[0].

GNU ld and gold don't need to do anything.
LLD's aarch64 port never reserves space for .got .

If the ABI wants to say something, it can be non-normative text that GNU
ld uses .got[0].
.got.plt[0~2] are reserved by all of GNU ld, gold, and LLD.

>>
>> A bonus with the new asm approach: we drop reliance on compiler generating
>> PC-relative relocations to get the runtime address.
>
>that's true but it's nicer to maintain c code:
>
>e.g. we may build ld.so with -mcmodel=tiny and
>then "adr" is enough instead of "adrp+add".

Done.

>i think we already rely on hidden symbol access
>not to generate dynamic relocs in the early ld.so
>code.
>
>csu/libc-start.c already uses __ehdr_start from
>c code.

I have a separate patch 
https://sourceware.org/pipermail/libc-alpha/2021-August/129917.html
("elf: Unconditionally use __ehdr_start").

>>
>> [1]: From a psABI maintainer, https://bugs.llvm.org/show_bug.cgi?id=49672#c2
>> [2]: LLD's aarch64 port does not set _GLOBAL_OFFSET_TABLE_[0] to the
>> link-time address _DYNAMIC.
>> LLD is widely used on aarch64 Android and ChromeOS devices.  Software
>> just works without the need for _GLOBAL_OFFSET_TABLE_[0].
>
>the patch should be ok with c instead of asm.
>
>thanks.

Thanks for the comments.
v2: https://sourceware.org/pipermail/libc-alpha/2021-August/129966.html

>> ---
>>  sysdeps/aarch64/dl-machine.h | 28 +++++++++++++---------------
>>  1 file changed, 13 insertions(+), 15 deletions(-)
>>
>> diff --git a/sysdeps/aarch64/dl-machine.h b/sysdeps/aarch64/dl-machine.h
>> index d29d827ab3..bf251d2972 100644
>> --- a/sysdeps/aarch64/dl-machine.h
>> +++ b/sysdeps/aarch64/dl-machine.h
>> @@ -37,28 +37,26 @@ elf_machine_matches_host (const ElfW(Ehdr) *ehdr)
>>    return ehdr->e_machine == EM_AARCH64;
>>  }
>>
>> -/* Return the link-time address of _DYNAMIC.  Conveniently, this is the
>> -   first element of the GOT. */
>> -static inline ElfW(Addr) __attribute__ ((unused))
>> -elf_machine_dynamic (void)
>> -{
>> -  extern const ElfW(Addr) _GLOBAL_OFFSET_TABLE_[] attribute_hidden;
>> -  return _GLOBAL_OFFSET_TABLE_[0];
>> -}
>> -
>>  /* Return the run-time load address of the shared object.  */
>>
>>  static inline ElfW(Addr) __attribute__ ((unused))
>>  elf_machine_load_address (void)
>>  {
>> -  /* To figure out the load address we use the definition that for any symbol:
>> -     dynamic_addr(symbol) = static_addr(symbol) + load_addr
>> +  ElfW(Addr) addr;
>> +  asm ("adrp %0, __ehdr_start\n\t"
>> +       "add %0, %0, :lo12:__ehdr_start" : "=r"(addr));
>> +  return addr;
>> +}
>>
>> -    _DYNAMIC sysmbol is used here as its link-time address stored in
>> -    the special unrelocated first GOT entry.  */
>> +/* Return the link-time address of _DYNAMIC.  */
>>
>> -    extern ElfW(Dyn) _DYNAMIC[] attribute_hidden;
>> -    return (ElfW(Addr)) &_DYNAMIC - elf_machine_dynamic ();
>> +static inline ElfW(Addr) __attribute__ ((unused))
>> +elf_machine_dynamic (void)
>> +{
>> +  ElfW(Addr) addr;
>> +  asm ("adrp %0, _DYNAMIC\n\t"
>> +       "add %0, %0, :lo12:_DYNAMIC" : "=r"(addr));
>> +  return addr - elf_machine_load_address ();
>>  }
>>
>>  /* Set up the loaded object described by L so its unrelocated PLT
>> --
>> 2.32.0.605.g8dce9f2422-goog
>>